Multiple-input, multiple-output (MIMO) systems with multiple transmission modes

ABSTRACT

Techniques to transmit data on a number of transmission channels in a multi-channel communication system using multiple transmission schemes requiring less channel-state information (CSI). These schemes may include a partial-CSI transmission scheme that transmits a single data stream on each transmit antenna selected for use and a “beam-forming” transmission scheme that allocates all transmit power to a single transmission channel having the best performance. Each transmission scheme may provide good or near-optimum performance for a specific range of operating conditions (or operating SNRs). These multiple transmission schemes may then be combined in a piece-wise fashion to form a “multi-mode” transmission scheme that covers the full range of operating conditions supported by the MIMO system. The specific transmission scheme to be used for data transmission at any given moment would then be dependent on the specific operating condition experienced by the system at that moment.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a Continuation and claims priorityto patent application Ser. No. 10/085,456 entitled “Multiple-Input,Multiple-Output (MIMO) Systems with Multiple Transmission Modes,” filedFeb. 26, 2002, now allowed, and assigned to the assignee hereof andhereby expressly incorporated by reference herein.

BACKGROUND

1. Field

The present invention relates generally to data communication, and morespecifically to multi-channel communication systems (e.g.,multiple-input, multiple-output (MIMO) systems) with multipletransmission modes.

2. Background

In a wireless communication system, an RF modulated signal from atransmitter may reach a receiver via a number of propagation paths. Thecharacteristics of the propagation paths typically vary over time due toa number of factors such as fading and multipath. To provide diversityagainst deleterious path effects and improve performance, multipletransmit and receive antennas may be used. If the propagation pathsbetween the transmit and receive antennas are linearly independent(i.e., a transmission on one path is not formed as a linear combinationof the transmissions on other paths), which is generally true to atleast an extent, then the likelihood of correctly receiving a datatransmission increases as the number of antennas increases. Generally,diversity increases and performance improves as the number of transmitand receive antennas increases.

A multiple-input multiple-output (MIMO) communication system employsmultiple (N_(T)) transmit antennas and multiple (N_(R)) receive antennasfor data transmission. A MIMO channel formed by the N_(T) transmit andN_(R) receive antennas may be decomposed into N_(S) independentchannels, with N_(S)≦in {N_(T), N_(R)}. Each of the N_(S) independentchannels is also referred to as a spatial subchannel of the MIMO channeland corresponds to a dimension. The MIMO system can provide improvedperformance (e.g., increased transmission capacity) if the additionaldimensionalities created by the multiple transmit and receive antennasare utilized. For example, an independent data stream may be transmittedon each of the N_(S) spatial subchannels to increase system spectralefficiency.

The spatial subchannels of a MIMO system may experience differentchannel conditions (e.g., different fading and multipath effects) andmay achieve different signal-to-noise ratios (SNRs) for a given amountof transmit power. Consequently, the data rates that may be supported bythe spatial subchannels may be different from subchannel to subchannel,depending on the amount of transmit power allocated to the data streamsand their achieved SNRs. Since the channel conditions typically varywith time, the transmission capacities of the spatial subchannels alsovary with time.

A key challenge in a coded communication system is to effectivelyutilize the total transmit power, P_(tot), available at the transmitterfor data transmission on the N_(S) spatial subchannels based on thechannel conditions. Various schemes may be used to transmit data on thespatial subchannels. Each transmission scheme may require certain typesof information regarding the MIMO channel and may further be premised oncertain signal processing at the transmitter and receiver. In general,more complicated transmission schemes may be able to achieve spectralefficiency closer to optimum by allocating different amounts of transmitpower to spatial subchannels of different capabilities andpre-conditioning the data streams prior to transmission over thesesubchannels. However, these transmission schemes generally require moreinformation regarding the MIMO channel, which may be difficult to obtainat the receiver and also requires air-link resources to report to thetransmitter. Less complicated transmission schemes may provide goodperformance over only a limited range of operating conditions, but mayrequire less channel information.

There is therefore a need in the art for techniques to transmit data ina MIMO system to achieve high spectral efficiency and having reducedcomplexity.

SUMMARY

Techniques are provided herein to transmit data over the availabletransmission channels in multi-channel communication systems such thathigher overall system spectral efficiency and/or other benefits may beachieved. The transmission channels may correspond to the spatialsubchannels of a MIMO system, the frequency subchannels of an OFDMsystem, or the spatial subchannels of the frequency subchannels in aMIMO-OFDM system.

In an aspect, multiple transmission schemes are selectively used toprovide overall efficiency near or approaching optimum. Eachtransmission scheme is dependent on whether full or partialchannel-state information (CSI) (described below) is available at thetransmitter to process data prior to transmission over the transmissionchannels. For a partial-CSI transmission scheme, a data stream may betransmitted on each transmit antenna (e.g., at the peak transmit powerfor the antenna). All or only a subset of the N_(T) transmit antennasmay be used for data transmission at any given moment.

For a full-CSI transmission scheme, one or more data streams areprocessed at the transmitter based on full-CSI processing (or a variantthereof, as described below) and transmitted over the MIMO channel. Thefull-CSI transmission scheme includes a water-filling transmissionscheme, a “selective channel inversion” transmission scheme, a “uniform”transmission scheme, a “principal eigenmode beam-forming” transmissionscheme, and a “beam-steering” transmission scheme, all of which rely onfull-CSI processing at the transmitter. The water-filling transmissionscheme allocates more transmit power to transmission channels with lessnoise and less transmit power to more noisy channels. The water-fillingtransmission scheme is optimal and can achieve capacity. The selectivechannel inversion transmission scheme allocates transmit powernon-uniformly over selected ones of the transmission channels such thatthe post-detection SNRs are similar for the selected transmissionchannels. The uniform transmission scheme allocates the total transmitpower equally among all transmission channels, and the beam-formingtransmission scheme allocates all transmit power to a singletransmission channel having the best performance. The beam-steeringtransmission scheme uniformly allocates the total transmit power to alltransmit antennas used for transmitting a single data stream, but thedata stream is transmitted with the proper phases from these transmitantennas. In general, any number and type of transmission schemes may beemployed by a multi-mode MIMO system to provide improved overallperformance.

Each transmission scheme may provide good or near-optimum performancefor a specific range of operating conditions, which may be quantified byan operating signal-to-noise ratio (SNR). These multiple transmissionschemes of different types (i.e., based on partial CSI, full CSI, and soon) may then be combined in a piece-wise fashion to form a “multi-mode”transmission scheme that covers the full range of SNRs supported by theMIMO system. The specific transmission scheme to be used to transmitdata at any given moment would then be dependent on the specificoperating condition experienced by the system at that moment.

In a specific embodiment, a method is provided for transmitting data ona number of transmission channels in a multi-channel communicationsystem. In accordance with the method, the operating condition (e.g.,the operating SNR) of the system is initially determined, and a specifictransmission scheme is selected from among a number of possibletransmission schemes based on the determined operating condition and theamount of channel state information available at the transmitter. Eachof the transmission schemes may be designated for use for a respectiverange of operating SNRs. One or more data streams to be transmitted arethen determined based on the selected transmission scheme. The one ormore data streams are then processed based on the selected transmissionscheme and the available CSI. For example, the data rate and the codingand modulation scheme to use for each data stream may be determinedbased on the CSI. In one embodiment, the partial-CSI transmission schemeis selected for use if the operating SNR is above a threshold SNR, andthe beam-forming transmission scheme is selected for use if theoperating SNR is below the threshold SNR.

Various aspects and embodiments of the invention are described infurther detail below. The invention further provides methods,processors, transmitter units, receiver units, base stations, terminals,systems, and other apparatuses and elements that implement variousaspects, embodiments, and features of the invention, as described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 shows three plots of the efficiencies achieved for a 4×4 MIMOsystem using the water-filling, partial-CSI with MMSE-SC, andbeam-forming transmission schemes;

FIG. 2 is a flow diagram of an embodiment of a process for transmittingdata on the available transmission channels in a MIMO system based on amulti-mode transmission scheme; and

FIG. 3 is a block diagram of an embodiment of a transmitter system and areceiver system.

DETAILED DESCRIPTION

The data transmission techniques described herein may be used forvarious multi-channel communication systems. Such multi-channelcommunication systems include multiple-input multiple-output (MIMO)communication systems, orthogonal frequency division multiplexing (OFDM)communication systems, MIMO systems that utilize OFDM (i.e., MIMO-OFDMsystems), and others. The multi-channel communication systems may alsoimplement code division multiple access (CDMA), time division multipleaccess (TDMA), frequency division multiple access (FDMA), or some othermultiple access techniques. Multiple access communication systems cansupport concurrent communication with a number of terminals (i.e.,users). For clarity, certain aspects and embodiments of the inventionare described specifically for a MIMO system such as a multiple-antennawireless communication system.

A MIMO system employs multiple (N_(T)) transmit antennas and multiple(N_(R)) receive antennas for data transmission. A MIMO channel formed bythe N_(T) transmit and N_(R) receive antennas is referred to as aN_(R)×N_(T) MIMO system, and may be decomposed into N_(S) independentchannels, with N_(S)≦in {N_(T), N_(R)}. Each of the N_(S) independentchannels is also referred to as a spatial subchannel (or a transmissionchannel) of the MIMO channel. The number of spatial subchannels isdetermined by the number of eigenmodes for the MIMO channel, which inturn is dependent on a channel response matrix, H, that describes theresponse between the N_(T) transmit and N_(R) receive antennas.

The elements of the channel response matrix, H, are composed ofindependent Gaussian random variables, as follows: $\begin{matrix}{{\underset{\_}{H} = \begin{bmatrix}h_{1,1} & h_{1,2} & \ldots & h_{1,N_{T}} \\h_{2,1} & h_{2,2} & \ldots & h_{2,N_{T}} \\\vdots & \vdots & \quad & \vdots \\h_{N_{R},1} & h_{N_{R},2} & \ldots & h_{N_{R},N_{T}}\end{bmatrix}},} & {{Eq}\quad(1)}\end{matrix}$where h_(i,j) is the coupling (i.e., the complex gain) between the i-thtransmit antenna and the i-th receive antenna.

The model for the MIMO system may be expressed as:y=Hx+n,   Eq (2)where

-   -   y is the received vector, i.e., y=[y₁y₂ . . . y_(N) _(R) ]^(T),        where {y_(i)} is the entry received on the i-th received antenna        and iε{1, . . . , N_(R)};    -   x is the transmitted vector, i.e., x=[x₁x₂ . . . x_(N) _(T)        ]^(T), where {x_(j)} is the entry transmitted from the j-th        transmit antenna and jε{1, . . . , N_(T)};    -   H is the channel response matrix for the MIMO channel;    -   n is the additive white Gaussian noise (AWGN) with a mean vector        of 0 and a covariance matrix of Λ_(n)=σ²I, where 0 is a vector        of zeros, I is the identity matrix with ones along the diagonal        and zeros everywhere else, and σ² is the variance of the noise;        and    -   [.]^(T) denotes the transpose of [.].

For simplicity, the MIMO channel is assumed to be a flat-fading channel.In this case, the elements of the channel response matrix, H, arescalars, and the coupling, h_(i,j), between each transmit-receiveantenna pair can be represented by a single scalar value. However, thetechniques described herein may also be applied to a frequency selectivechannel having different channel gains at different frequencies. In sucha frequency selective channel, the operating bandwidth may be dividedinto a number of (equal or unequal) frequency bands such that each bandmay be considered as a flat-fading channel. A particular transmissionscheme may then be used for each of the frequency bands, subject tocertain constraints such as maintaining the total transmit power usedfor all frequency bands of a given transmit antenna to within the peaktransmit power of the antenna. In this way, the channel response of theindividual frequency bands may be considered in the data transmission.

Due to scattering in the propagation environment, N_(S) data streamstransmitted from the N_(T) transmit antennas interfere with each otherat the receiver. Multiple data streams may be transmitted on the spatialsubchannels using channel-state information (CSI), which is descriptiveof the characteristics of the MIMO channel. CSI may be categorized aseither “full CSI” or “partial CSI.” Full CSI includes sufficientcharacterization (e.g., amplitude and phase) for the propagation pathbetween each transmit-receive antenna pair in a (N_(R)×N_(T)) MIMOmatrix as well as relevant modulation/coding information for the datastreams. Partial CSI may comprise, for example, thesignal-to-noise-and-interference ratios (SNRs) of the data streams. Thefull or partial CSI may be determined at the receiver (e.g., based onthe received pilot and/or traffic data) and reported to the transmitter.

Different transmission schemes may be used depending on whether full orpartial CSI is available at the transmitter. When full CSI is available,the data streams may be transmitted on the eigenmodes of the MIMOchannel. This may be achieved by pre-conditioning the data streams atthe transmitter based on a set of (right) eigenvectors derived from thechannel response matrix, H, as described below. At the receiver, thetransmitted data streams may be recovered by multiplying the receivedsymbol streams with a set of (left) eigenvectors, which are also derivedbased on the matrix H. The full-CSI (or eigenmode) transmission schemeis thus dependent on knowledge of the channel response matrix, H. Thefull-CSI transmission scheme and variants of this scheme are describedin further detail below.

When only partial CSI is available, the data streams may be transmittedfrom the transmit antennas without pre-conditioning at the transmitter.At the receiver, the received symbol streams are processed in accordancewith a particular spatial or space-time receiver processing technique toattempt to separate out the data streams. The partial-CSI transmissionscheme is also described in further detail below.

For both the full and partial CSI transmission schemes, the data rateand the coding and modulation for each data stream are dependent on theSNR achieved for the data stream. The SNR of each data stream may beestimated at the receiver. Information descriptive of either theestimated SNR or the particular coding and modulation scheme to be usedfor each data stream may then be provided to the transmitter and used toprocess the data stream prior to transmission over the MIMO channel.

When full CSI is available, one technique for eliminating or reducingthe interference among the data streams is to “diagonalize” the MIMOchannel such that the data streams are effectively transmitted onorthogonal spatial subchannels. One technique for diagonalizing the MIMOchannel is to perform singular value decomposition on the channelresponse matrix, H, which can be expressed as:H=UDV^(H),   Eq (3)where U is an N_(R)×N_(R) unitary matrix (i.e., U^(H)U=I);

-   -   D is an N_(R)×N_(T) matrix;    -   V is an N_(T)×N_(T) unitary matrix; and    -   “^(H)” denotes the complex transpose of a matrix.        The diagonal entries of matrix D are the square roots of the        eigenvalues of G=H^(H)H, denoted by λ_(i) and iε{1, . . . N_(S)}        where N_(S)≦in{N_(T),N_(R)} is the number of resolvable data        streams. All non-diagonal entries of D are zero.

The diagonal matrix D thus contains non-negative real values along thediagonal and zeros elsewhere, where the non-negative real values ared_(i)={square root}{square root over (λ_(i))}. The d_(i) are referred toas the singular values of the channel response matrix, H. The singularvalue decomposition is a matrix operation known in the art and describedin various references. One such reference is a book by Gilbert Strangentitled “Linear Algebra and Its Applications,” Second Edition, AcademicPress, 1980, which is incorporated herein by reference.

The singular value decomposition decomposes the channel response matrix,H, into two unitary matrices, U and V, and the diagonal matrix, D.Matrix D is descriptive of the eigenmodes of the MIMO channel, whichcorrespond to the spatial subchannels. The unitary matrices, U and V,include “steering” vectors (or left and right eigenvectors,respectively) for the receiver and transmitter, respectively, which maybe used to diagonalize the MIMO channel. Specifically, to diagonalizethe MIMO channel, a signal vector, s, may be pre-multiplied with thematrix, V, at the transmitter to provide the transmitted vector, x, asfollows:x=Vs.   Eq (4)This vector x is then transmitted over the MIMO channel to the receiver.

At the receiver, the received vector, y=Hx+n, may be pre-multiplied withthe matrix, U^(H), to obtain a recovered vector, r, as follows:$\begin{matrix}\begin{matrix}{\underset{\_}{r} = {{{\underset{\_}{U}}^{H}\underset{\_}{H}\underset{\_}{V}\underset{\_}{s}} + {{\underset{\_}{U}}^{H}\underset{\_}{n}}}} \\{{= {{\underset{\_}{D}\underset{\_}{s}} + \hat{\underset{\_}{n}}}},}\end{matrix} & {{Eq}\quad(5)}\end{matrix}$where {circumflex over (n)} is simply a rotation of n, resulting inadditive white Gaussian noise with the same mean vector and covariancematrix as n.

As shown in equation (4), the pre-multiplication of the signal vector,s, by the matrix V and the pre-multiplication of the received vector, y,by the matrix U^(H) result in an effective diagonal channel, D, which isthe transfer function between the signal vector, s, and the recoveredvector, r. As a result, the MIMO channel is decomposed into N_(S)independent, non-interfering, orthogonal, and parallel channels. Theseindependent channels are also referred to as the spatial subchannels ofthe MIMO channel. Spatial subchannel i or eigenmode i has a gain that isequal to the eigenvalue, λ_(i), where iεI and set I is defined as I={1,. . . , N_(S)}. Diagonalization of the MIMO channel to obtain N_(S)orthogonal spatial subchannels can be achieved if the transmitter isprovided with an estimate of the channel response matrix, H.

For the full-CSI transmission scheme, one data stream may be transmittedon each of the N_(S) spatial subchannels or eigenmodes. For each spatialsubchannel to be used for data transmission, the transmitter is providedthe (right) eigenvector as well as relevant modulation/codinginformation for that subchannel. Thus, if all N_(S) spatial subchannelsare used for data transmission, then the transmitter is provided witheither the channel response matrix, H, or the unitary matrix, V, and therelevant modulation/coding information.

For the partial-CSI transmission scheme, one data stream may betransmitted on each of the N_(T) transmit antennas (assuming that H is afull-rank matrix and N_(S)=N_(T)=N_(R)). For the partial-CSItransmission scheme, the transmitter needs not be provided with thechannel response matrix, H, or the unitary matrix, V, since the datastreams are not pre-conditioned prior to transmission over the MIMOchannel.

For the partial-CSI transmission scheme, various receiver processingtechniques may be used at the receiver to process the received symbolstreams to separate out the transmitted data streams. These receiverprocessing techniques include spatial receiver processing techniques(which may be used for a non-dispersive channel with flat fading) andspace-time receiver processing techniques (which may be used for adispersive channel with frequency selective fading). The spatialreceiver processing techniques include a channel correlation matrixinversion (CCMI) technique and a minimum mean square error (MMSE)technique. The space-time receiver processing techniques include an MMSElinear equalizer (MMSE-LE), a decision feedback equalizer (DFE), and amaximum-likelihood sequence estimator (MLSE). In general, these spatialand space-time receiver processing techniques require an estimate of thechannel response at the receiver (but not the transmitter).

A “successive nulling/equalization and interference cancellation”receiver processing technique (which is also referred to as a“successive interference cancellation” or “successive cancellation”receiver processing technique) may also be used in conjunction with anyof the spatial or space-time technique described above to provideimproved performance. For example, successive interference cancellationmay be used with MMSE (i.e., MMSE-SC) to process the received symbolstreams at the receiver to recover the data streams.

The various receiver processing techniques are described in detail inU.S. patent Publication No. US-2003-0125040-A1, entitled“Multiple-Access Multiple-Input Multiple-Output (MIMO) CommunicationSystem,” filed Nov. 6, 2001; U.S. Pat. No. 6,785,341, entitled “Methodand Apparatus for Processing Data in a Multiple-Input Multiple-Output(MIMO) Communication System Utilizing Channel State Information,” filedMay 11, 2001 and issued Aug. 31, 2004; U.S. Pat. No. 6,771,706, filedMar. 23, 2001 and issued Aug. 3, 2004, and U.S. patent Publication No.US-2003-0003880-A1, filed Sep. 18, 2001, both entitled “Method andApparatus for Utilizing Channel State Information in a WirelessCommunication System.” These applications are all assigned to theassignee of the present application and incorporated herein byreference.

The full-CSI transmission scheme requires more information regarding theMIMO channel (e.g., the matrix H), which is typically derived at thereceiver and reported back to the transmitter. Thus, there is higheroverhead cost associated with implementing the full-CSI transmissionscheme. The partial-CSI transmission scheme does not require thisadditional information.

In a typical MIMO system, a peak transmit power of P_(max) may beimposed on each of the N_(T) transmit antennas. In this case, the totaltransmit power, P_(tot), available at the transmitter for all N_(T)transmit antennas may be expressed as:P _(tot) =N _(T) ·P _(max).   Eq (6)The total transmit power, P_(tot), may be allocated to the data streamsbased on various schemes.

A number of transmission schemes may be derived based on the full-CSItransmission scheme, with each such transmission scheme being dependent(in part) on how the total transmit power is allocated to theeigenmodes. These transmission schemes include a “water-filling”transmission scheme, a “selective channel inversion” transmissionscheme, a “uniform” transmission scheme, a “principal eigenmodebeam-forming” (or simply, “beam-forming”) transmission scheme, and a“beam-steering” transmission scheme. Fewer, additional, and/or differenttransmission schemes may also be considered and are within the scope ofthe invention. For the water-filling transmission scheme, the totaltransmit power is allocated such that more transmit power is allocatedto transmission channels with less noise and less transmit power isallocated to more noisy channels. For the selective channel inversiontransmission scheme, the total transmit power is non-uniformly allocatedto selected ones of the transmission channels such that they achieveapproximately similar post-detection SNRs. For the uniform transmissionscheme, the total transmit power is allocated equally among thetransmission channels. For the beam-forming transmission scheme, alltransmit power is allocated to a single transmission channel having thebest performance. And for the beam-steering transmission scheme, thetotal transmit power is uniformly allocated to all transmit antennasused for transmitting a single data stream, but the data stream istransmitted with the proper phases from these transmit antennas. Thesetransmission schemes rely on full-CSI processing (or a variant thereof)at the transmitter for each transmission channel selected for use. Thesevarious full-CSI based transmission schemes are described in furtherdetail below.

The water-filling transmission scheme allocates the total transmitpower, P_(tot), to the eigenmodes such that transmission capacity (i.e.,spectral efficiency) is maximized. The water-filling power allocation isanalogous to pouring a fixed amount of water into a vessel with anirregular bottom, where each eigenmode corresponds to a point on thebottom of the vessel, and the elevation of the bottom at any given pointcorresponds to the inverse of the signal-to-noise ratio (SNR) associatedwith that eigenmode. A low elevation thus corresponds to a high SNR and,conversely, a high elevation corresponds to a low SNR. The totaltransmit power, P_(tot), is then “poured” into the vessel such that thelower points in the vessel (i.e., higher SNRs) are filled first, and thehigher points (i.e., lower SNRs) are filled later. The powerdistribution is dependent on the total transmit power, P_(tot), and thedepth of the vessel over the bottom surface. The water surface level forthe vessel after all of the total transmit power has been poured isconstant over all points in the vessel. The points with elevations abovethe water surface level are not filled (i.e., eigenmodes with SNRs belowa particular threshold are not used). The water-filling distribution isdescribed by Robert G. Gallager, in “Information Theory and ReliableCommunication,” John Wiley and Sons, 1968, which is incorporated hereinby reference.

Capacity is defined as the highest spectral efficiency at whichinformation can be communicated with an arbitrarily low probability oferror, and is typically given in unit of bits per second per Hertz(bps/Hz). The capacity for one Gaussian channel with an SNR of γ may beexpressed as:C=log₂(1+γ).   Eq (7)

For a MIMO system with limited total transmit power of P_(tot), thewater-filling transmission scheme can optimally allocate the totaltransmit power to the N_(S) spatial subchannels such that capacity isachieved. The water-filling transmission scheme distributes the totaltransmit power, P_(tot), over the eigenmodes in such a way that theeigenmode with the lowest noise variance (i.e., the highest SNR)receives the greatest fraction of the total power. The amount of powerallocated to eigenmode i as a result of water filling is denoted byP_(i), for iεI, where $\begin{matrix}{P_{tot} = {\sum\limits_{i \in I}\quad{P_{i}.}}} & {{Eq}\quad(8)}\end{matrix}$

Based on the allocated transmit power of P_(i) for eigenmode i, for iεI,the effective SNR for eigenmode i, γ_(i), may be expressed as:$\begin{matrix}{{\gamma_{i} = \frac{P_{i} \cdot \lambda_{i}}{\sigma^{2}}},} & {{Eq}\quad(9)}\end{matrix}$where λ_(i) is the channel gain on sub-channel i and σ₂ is the noisevariance for the MIMO channel, assuming the same noise variance over allsub-channels. The capacity achieved by the water-filling transmissionscheme for the N_(S) spatial subchannels may then be expressed as:$\begin{matrix}{C = {\sum\limits_{i = 1}^{N_{S}}\quad{{\log_{2}\left( {1 + \gamma_{i}} \right)}.}}} & {{Eq}\quad(10)}\end{matrix}$

The spectral efficiency of each eigenmode may be determined based on aparticular monotonically increasing function in SNR. One function thatmay be used for spectral efficiency is the capacity function shown inequation (7). In this case, the spectral efficiency for eigenmode i,ρ_(i), may be expressed as:ρ_(i)=log₂(1+γ_(i)).   Eq (11)The total spectral efficiency of the system is the summation of thespectral efficiencies of all eigenmodes, each of which may be determinedas shown in equation (11).

A specific algorithm for performing water-filling power allocation for aMIMO-OFDM system is described in U.S. patent Publication No.US-2003-0072379-A1, entitled “Method and Apparatus for Determining PowerAllocation in a MIMO Communication System,” filed Oct. 15, 2001. Aspecific algorithm for performing water-filling power allocation for aMIMO system is described in U.S. patent Publication No.US-2003-0139196-A1, entitled “Reallocation of Excess Power in aMulti-Channel Communication System,” filed Jan. 23, 2002. Both of theseapplications are assigned to the assignee of the present application andincorporated herein by reference.

The selective channel inversion transmission scheme allocates the totaltransmit power, P_(tot), non-uniformly over selected ones of the N_(S)eigenmodes such that the post-detection SNRs of the data streamstransmitted on these selected eigenmodes are approximately similar. Forthis transmission scheme, poor eigenmodes are not selected for use. Thesimilar post-detection SNRs allow the same coding and modulation schemeto be used for all data streams, which can simplify the processing atboth the transmitter and receiver.

Techniques for allocating transmit power to achieve similarpost-detection SNRs are described in U.S. patent Publication No.US-2003-0048856-A1, filed May 17, 2001, Publication No.US-2003-0112880-A1, filed Jun. 14, 2001, and U.S. Pat. No. 6,751,187,filed Jun. 26, 2001, all three entitled “Method and Apparatus forProcessing Data for Transmission in a Multi-Channel Communication SystemUsing Selective Channel Inversion,” assigned to the assignee of thepresent application and incorporated herein by reference.

The uniform transmission scheme allocates the total transmit power,P_(tot), equally over all N_(S) eigenmodes. This can be achieved byallocating P_(tot)/N_(S) to each eigenmode. Based on the allocatedtransmit power of P_(tot)/N_(S) for eigenmode i, for iεI, the effectiveSNR for eigenmode i, {circumflex over (γ)}_(i), may be expressed as:$\begin{matrix}{{\hat{\gamma}}_{i} = {\frac{P_{tot} \cdot \lambda_{i}}{N_{S} \cdot \sigma^{2}}.}} & {{Eq}\quad(12)}\end{matrix}$

The spectral efficiency achieved by the uniform transmission scheme forthe N_(S) spatial subchannels may then be expressed as: $\begin{matrix}{\hat{C} = {\sum\limits_{i = 1}^{N_{S}}\quad{{\log_{2}\left( {1 + {\hat{\gamma}}_{i}} \right)}.}}} & {{Eq}\quad(13)}\end{matrix}$

The beam-forming transmission scheme allocates the total transmit power,P_(tot), to a single eigenmode. In order to approach capacity, the totaltransmit power is allocated to the eigenmode corresponding to thehighest eigenvalue, λ_(max). This then maximizes the SNR given theconstraint of using a single eigenmode for data transmission. Theeffective SNR for the single (best) eigenmode may be expressed as:$\begin{matrix}{\overset{\sim}{\gamma} = {\frac{P_{tot} \cdot \lambda_{\max}}{\sigma^{2}}.}} & {{Eq}\quad(14)}\end{matrix}$The eigenvalues, λ_(i), for iεI, may be ordered in decreasing order. Inthis case, λ₁ is then the highest eigenvalue (i.e., λ₁=λ_(max))

The spectral efficiency achieved by the beam-forming transmission schemefor the N_(S) spatial subchannels may be expressed as:{tilde over (C)}=log₂(1+{tilde over (γ)}).   Eq (15)Since only one eigenmode is used for data transmission, equation (15)does not include a summation over the N_(S) spatial subchannels, as isthe case for equations (10) and (13) for the water-filling and uniformtransmission schemes, respectively.

Although the beam-forming transmission scheme is based on full-CSIprocessing at the transmitter, less channel state information isrequired to implement this transmission scheme since only one eigenmodeis used for data transmission. In particular, only one singular vectorcorresponding to the selected eigenmode needs to be provided to thetransmitter, which then uses this vector to pre-condition the datastream prior to transmission over the MIMO channel. This singular vectormay be derived at the receiver based on the channel response matrix, H,and provided to the transmitter.

The beam-steering transmission scheme allocates the total transmitpower, P_(tot), uniformly to all transmit antennas used for transmittinga single data stream. At low SNRs, the water-filling transmission schemetends to allocate a large portion of the total transmit power to aprincipal eigenmode, which is the eigenmode corresponding to the highesteigenvalue, λ_(max). For the beam-forming transmission scheme, a singledata stream is transmitted on the principal eigenmode, and this datastream is scaled by a complex gain value associated with each transmitantenna used for data transmission, as determined by the singular vectorcorresponding to the principal eigenmode. The magnitude of the complexgain value determines the amount of transmit power to be used for thetransmit antenna.

The beam-steering transmission scheme is similar to the beam-formingtransmission scheme and transmits a single data stream over the MIMOchannel. However, since only one data stream is transmitted, it is notnecessary to orthogonalize the transmission channels or to restrict thetransmission of this data stream on a single transmission channelcorresponding to the principal eigenmode. The beam-steering transmissionscheme relies on the principal eigenmode, which achieves the bestperformance, but allocates the total transmit power uniformly to alltransmit antennas used for data transmission. In this way, highertransmit power is utilized for the data stream, which may result inimproved performance.

For the beam-steering transmission scheme, the eigenmode correspondingto the highest eigenvalue, λ_(max), is initially identified (e.g., atthe receiver), and the singular vector corresponding to this eigenmodeis determined. This singular vector includes N_(T) complex values forthe complex gains to be used for the N_(T) transmit antenna. Thebeam-steering transmission scheme transmits the single data stream fromthe N_(T) transmit antennas at full power but with the proper phases,which are the phases of the N_(T) complex gain values in the singularvector. Thus, only the phases of the N_(T) elements in the singularvector need to be provided to the transmitter. The data stream is thentransmitted from the N_(T) transmit antennas at normalized (e.g., full)transmit power but with the phases for the principal eigenmode. Thisthen allows the transmissions from the N_(T) transmit antennas to beconstructively (or coherently) combined at the receiver, which canprovide improved performance.

A number of transmission schemes may also be derived based on thepartial-CSI transmission scheme. In one scheme, the peak transmit power,P_(max), is used for each data stream, and N_(T) data streams aretransmitted from the N_(T) transmit antennas using partial-CSIprocessing (i.e., no pre-conditioning at the transmitter). In anotherscheme (which may be referred to as the “selective partial-CSI scheme”),only selected ones of the N_(T) transmit antennas are used for datatransmission, and one data stream is transmitted from each selectedtransmit antenna (e.g., using peak transmit power). Other variants ofthe partial-CSI transmission scheme may also be formed.

A number of transmission schemes may thus be used for data transmission.Each transmission scheme is dependent on whether the full or partial CSIis available at the transmitter.

FIG. 1 shows three plots of the efficiencies achieved for an example 4×4MIMO system using the water-filling, uniform, and beam-formingtransmission schemes described above. The efficiencies are determinedbased on an assumption of an uncorrelated, complex Gaussian channelmodel whereby the MIMO channel experiences additive white Gaussian noise(AWGN) but no other interference.

For the water-filling transmission scheme, a large number of randomchannel sets (i.e., sets of spatial subchannels with random eigenvalues)are initially generated. These channel sets are then evaluated forvarious discrete values of noise variance σ², where each noise variancevalue corresponds to a particular “operating” SNR, as described below.For each channel set, water-filling is used to allocate the totaltransmit power to the spatial subchannels in the set based on thesubchannels' eigenvalues and for various noise variance values. Theeffective SNRs of the spatial subchannels in each set are dependent onthe subchannels' eigenvalues, the allocated transmit power, and thenoise variance, and may be determined as shown in equation (9). Theefficiency of each channel set is then determined for each noisevariance value, as shown in equation (10). A statistical average of theefficiency of all channel sets is then obtained for each noise variancevalue.

For the beam-forming transmission scheme, the same randomly-generatedchannel sets are evaluated, except that only the eigenmode correspondingto the highest eigenvalue is selected for use. Each channel set issimilarly evaluated for various discrete values of noise variance σ²,and a statistical average of the efficiency of these channel sets isobtained for each noise variance value. For the uniform transmissionscheme, the total transmit power is uniformly allocated to theeigenmodes in each randomly-generated channel set. Each set is alsoevaluated for various discrete noise variance values, and thestatistical average of the efficiency of the channel sets is obtainedfor each noise variance value.

As shown in FIG. 1, the efficiency achieved by each transmission schemeis plotted versus operating SNR. The operating SNR is the inverse of thepower of the additive white Gaussian noise at the receiver and isdefined as: $\begin{matrix}{\gamma_{op} = {\frac{1}{\sigma^{2}}.}} & {{Eq}\quad(16)}\end{matrix}$

The operating SNR is a measure of the operating condition of the MIMOchannel. As shown in equation (16), the operating SNR and the noisevariance σ² are inversely related. The efficiencies obtained for variousnoise variance values for each transmission scheme may thus be plottedversus operating SNR instead of noise variance for ease ofunderstanding. As shown in FIG. 1, the spectral efficiency of thewater-filling transmission scheme shown by plot 112 is the best of thethree transmission schemes and can be shown to be equal to capacity. Theefficiencies for the uniform and beam-forming transmission schemes areshown by plots 114 and 116, respectively. At low SNRs, the beam-formingtransmission scheme is close to optimum (i.e., the water-fillingtransmission scheme) because only one eigenmode is often active at theseSNRs. At high SNRs, the uniform transmission scheme approaches theoptimum performance of the water-filling transmission scheme.

The efficiency for the uniform transmission scheme shown by plot 114 isachieved using full-CSI processing at the transmitter. In particular,equation (13) indicates that the efficiency, Ĉ, may be achieved based onthe effective SNRs of {circumflex over (γ)}_(i) for the spatialsubchannels, and equation (12) suggests full CSI (e.g., the channelresponse matrix, H) is needed to derive the eigenvalues, λ_(i), whichare then used to determine the effective SNRs. However, it can be shownthat the efficiency, Ĉ, can also be obtained for the partial-CSItransmission scheme if the MMSE-SC receiver processing technique is usedat the receiver to process and recover the transmitted data streams. Ifsome other receiver processing technique is used at the receiver insteadof the MMSE-SC technique, then the efficiency for the partial-CSItransmission scheme may be less than that shown in equation (13).

To achieve the optimum efficiency of the water-filling transmissionscheme, the transmitter needs full knowledge of the MIMO channel, i.e.,full CSI. Full CSI may be provided by the channel response matrix, H,and the noise variance, σ². The channel response matrix, H, may then beevaluated (e.g., using singular value decomposition) to determine theeigenmodes and eigenvalues of the matrix G=H^(H)H. The total transmitpower may then be allocated to the eigenmodes based on the eigenvaluesand the noise variance using the water-filling transmission scheme.

The water-filling transmission scheme may not be preferred or availablefor some MIMO systems due to various factors. First, full CSI may bedifficult to obtain (e.g., typically at the receiver) because this wouldrequire measurement of the channel gain between each transmit-receiveantenna pair. Second, additional air-link resources would be required toreport the full CSI for all eigenmodes from the receiver back to thetransmitter. Third, higher accuracy (i.e., more bits) may be required torepresent the channel gains since any error translates to acorresponding loss in orthogonality. Fourth, the channel gains are moresensitive to measurement and reporting delays if the MIMO channel istime-varying. These factors may curtail the use of the water-fillingtransmission scheme for some MIMO systems.

In an aspect, multiple transmission schemes having seemingly sub-optimalefficiencies, but which require less CSI to implement, are selectivelyused to provide overall efficiency near or approaching optimum. Thesesub-optimal transmission schemes may include, for example, thepartial-CSI transmission scheme (e.g., with MMSE-SC receiver processing)and the beam-forming transmission scheme (or the beam-steeringtransmission scheme) described above. Different and/or additionaltransmission schemes may also be used, and this is within the scope ofthe invention.

The “near-optimum” overall performance may be attained for thesub-optimal transmission schemes by providing the transmitter withnecessary CSI. This CSI may comprise “post-processed” SNRs of the datatransmissions on the N_(S) eigenmodes, as determined at the receiverafter performing spatial receiver processing, as described below. Thepost-processed SNRs may be used by the transmitter to select (1) theparticular transmission scheme from among multiple possible transmissionschemes to use for data transmission, and (2) the data rate and thecoding and modulation scheme to use for each data stream.

For the beam-forming transmission scheme, a singular vector, v, ofscalar values associated with the selected eigenmode (e.g.,corresponding to the highest eigenvalue, λ_(max)) is also provided tothe transmitter and used to beam-form the data transmission on thiseigenmode. The receiver can pre-multiply the received vector, y, with acorresponding singular vector, u, to recover the transmitted datastream.

To achieve high performance using only partial CSI, the receiver mayemploy the MMSE-SC receiver processing technique, which can yield thespectral efficiency achieved by the uniform transmission scheme, whichrequires full CSI.

As SNR is reduced, the water-filling transmission scheme tends toallocate a greater fraction of the total transmit power to the principaleigenmode having better performance. At some threshold SNR, γ_(th), agood strategy is to allocate the total transmit power to the eigenmodecorresponding to the maximum eigenvalue. As shown in FIG. 1, theperformance of the beam-forming transmission scheme (plot 116)approaches the optimum efficiency of the water-filling transmissionscheme (plot 112) at increasingly lower SNRs. Conversely, as SNR isincreased, the difference in power per eigenmode allocated by thewater-filling transmission scheme relative to the total power decreases,and the power allocation appears more uniform. As SNR increases, thenoise variance, σ², decreases, and the elevations of the differenteigenmodes (which are determined as σ²/λ_(i)) go down lower. As alsoshown in FIG. 1, the performance of the uniform transmission scheme andthe partial-CSI with MMSE-SC scheme (plot 114) approaches the efficiencyof the water-filling transmission scheme at increasingly higher SNRs.

The partial-CSI transmission scheme, with MMSE-SC (or an equivalentscheme) at the receiver, can achieve the spectral efficiency of theuniform transmission scheme under certain conditions, but without theextra “cost” associated with full CSI required by the uniformtransmission scheme. As seen in FIG, 1, the spectral efficiency of thepartial-CSI with MMSE-SC scheme decreases substantially at low SNRs.

In an aspect, a MIMO system may advantageously employ multipletransmission schemes (e.g., the partial-CSI with MMSE-SC scheme and thebeam-forming or beam-steering transmission scheme) to provide improvedperformance over a wider range of SNRs. Each transmission scheme used bythe MIMO system may correspond to a respective mode of operation. The“multi-mode” MIMO system may then switch between the various supportedmodes of operation (e.g., the partial CSI and beam-forming modes)depending on the channel (or operating) conditions. In this way, thetransmission scheme that provides the best performance for a givenoperating condition may be selected for use to provide high performance.

FIG. 1 also shows a plot 120 (represented by the circled dots) for theefficiency achieved by a multi-mode MIMO system that employs both thepartial-CSI (with MMSE-SC) and beam-forming transmission schemes. Thespectral efficiency, C_(mm), achieved by this multi-mode system at anyoperating SNR may be expressed as:C _(mm)=max(Ĉ,{tilde over (C)}),   Eq (17)where Ĉ and {tilde over (C)} are the spectral efficiencies for thepartial-CSI employing MMSE-SC and the beam-forming transmission schemesgiven by equations (13) and (15), respectively. The maximum loss inefficiency encountered by the use of these two transmission schemesoccurs near an operating SNR of γ_(op)=0 dB, and is approximately 1.75dB, for this example MIMO system employing four transmit and fourreceive antennas. With the implementation of this multi-mode system, theloss in efficiency reduces at both low and high SNRs. As shown in FIG.1, the beam-forming transmission scheme may be used to provide extendedrange of operation (i.e., covering low SNRs) for the MIMO system. Theperformance for the partial-CSI with MMSE-SC and beam-formingtransmission schemes assumes an uncorrelated channel model. As thechannel gets more correlated, fewer data streams can be resolved at thereceiver. Consequently, the intersection of partial-CSI with MMSE-SC andbeam-forming transmission schemes shifts to a higher SNR, and thebeam-forming transmission scheme becomes the chosen mode of operationfor a greater range of SNRs at the low end.

Some of the transmission schemes described above (e.g., thebeam-steering transmission scheme) may also be employed by the system.Other transmission schemes may also be used, and this is within thescope of the invention. For example, a “selective eigenmode”transmission scheme may be used to uniformly allocate the total transmitpower among a subset of the N_(S) eigenmodes. This scheme may beselected for use, for example, if two or more eigenmodes achieveeffective SNRs above some threshold SNR. A “selective partial-CSI”transmission scheme may also be used whereby only some of the transmitantennas are used for data transmission and the remaining transmitantennas would then be turned off.

Variations to the transmission schemes described herein may also beimplemented, and this is within the scope of the invention. For example,a transmit diversity scheme may be used whereby a single data stream istransmitted over all N_(T) transmit antennas at full power without anybeam-forming. For the partial CSI with MMSE-SC scheme, which yields thesame spectral efficiency as the uniform transmission scheme undercertain conditions, the actual transmit power used for a given datastream may be adjusted to be the minimum amount needed for a given(quantized) data rate.

In general, any number and type of transmission schemes may be employedby a multi-mode MIMO system to provide improved overall performance.Each transmission scheme may provide good or near-optimum performanceover some operating conditions (e.g., a specific range of SNRs). Thesemultiple transmission schemes may then be combined in a piece-wisefashion to form a multi-mode transmission scheme that covers alloperating conditions (e.g., the full range of SNRs) supported by theMIMO system.

FIG. 1 shows the spectral efficiency achieved by each of the three(water-filling, uniform, and beam-forming) transmission schemes, whichis generally true on average. However, the spectral efficiencies shownin FIG. 1 do not take into account losses related to quantization of thesingular vector, correlation in the channel, and other factors.

In general, the spectral efficiency achieved by each transmission schemeat any given time instant is a function of the operating SNR as well asthe channel at that instant. Thus, to achieve high performance, thechannel (and other factors) may be taken into account when selectingwhich transmission scheme to use. For slowly-varying channels, theinstantaneous channel estimate may be used to evaluate the possibletransmission schemes. For faster-varying channels, a time average of thechannel may be obtained and used as the channel estimate.

The specific transmission scheme to be used for data transmission at anygiven time instance may be selected in various ways. The transmissionscheme selection may be dependent on the specific operating conditionsexperienced by the system at that time instant and may further take intoaccount other factors. Several methods for selecting transmission schemeare described below.

In one method for selecting transmission scheme, the specifictransmission scheme to use for data transmission is selected basedsolely on the operating SNR. This method is simple to implement and mayprovide the desired level of performance.

In another method for selecting transmission scheme, the receiverevaluates each of the possible transmission schemes, and selects thetransmission scheme that yields the highest spectral efficiency. For asystem that only supports the beam-forming and partial-CSI transmissionschemes, the receiver can evaluate the performance achieved for thebeam-forming transmission scheme (using a quantized singular vector) andthe partial-CSI with MMSE-SC (or some other receiver processing) scheme.The receiver can then select the transmission scheme that yields thehighest throughput and provide this information to the transmitter.

In yet another method for selecting transmission scheme, the receiverevaluates each of the possible transmission schemes, and selects one ofthe transmission schemes based on the achievable spectral efficiency andother considerations. For example, the receiver can evaluate theperformance achieved for the beam-forming and uniform transmissionschemes. If the ratio of the spectral efficiencies (e.g., beam-formingspectral efficiency/uniform spectral efficiency) is greater than somethreshold value, then the beam-forming transmission scheme may beselected for use. Otherwise, the partial-CSI transmission scheme may beselected. The uniform transmission scheme is typically “easier” (lesscomputationally expensive, faster) to evaluate than the partial-CSI withMMSE-SC scheme, and may be used as a substitute (the receiver canevaluate the performance of the uniform transmission scheme since it maybe able to obtain the full CSI). However, due to some implementationlosses, the spectral efficiency of the uniform transmission scheme maynot be exactly equal to that of the partial-CSI with MMSE-SC scheme.Furthermore, there is quantization loss associated with the beam-formingtransmission scheme. Thus, the threshold value may be selected to takethese various factors into account.

Other methods for selecting the specific transmission scheme to use atany given time instant may also be devised, and this is within the scopeof the invention. In general, any number of the possible transmissionschemes (or their equivalents) may be evaluated, and the specifictransmission scheme to be used for data transmission may be selectedbased on various factors such as (1) the achievable spectral efficiency,(2) the estimated implementation losses, and so on.

FIG. 2 is a flow diagram of an embodiment of a process 200 fortransmitting data in a MIMO system based on a multi-mode transmissionscheme. Initially, the operating condition of the MIMO system isdetermined (step 212). The operating condition may be quantified by theoperating SNR, which may be determined based on the noise variance, asshown in equation (16), and/or other factors. The operating conditionmay be estimated based on a pilot transmitted along with the data, as isknown in the art.

A specific transmission scheme is then selected from among multipletransmission schemes based on the determined operating condition (step214). As noted above, any number of transmission schemes may besupported by the MIMO system. The specific transmission scheme to usefor data transmission may be determined by comparing the operating SNRagainst one or more threshold SNRs. If the MIMO system supports only thepartial-CSI and beam-forming transmission schemes, then the partial-CSIscheme may be selected if the operating SNR is equal to or greater thanthe threshold SNR, γ_(th), and the beam-forming transmission scheme maybe selected if the operating SNR is less than the threshold SNR.

The number of data streams to be transmitted is then determined, withthe number being dependent on the selected transmission scheme (step216). For example, a single data stream may be transmitted on a singleeigenmode corresponding to the highest eigenvalue for the beam-formingtransmission scheme, and N_(T) data streams may be transmitted on N_(T)transmit antennas for the partial-CSI transmission scheme. The totaltransmit power, P_(tot), available to the system is then allocated tothe one or more data streams based on the selected transmission scheme(step 218). The one or more data streams are then processed based on theselected transmission scheme and in accordance with the allocatedtransmit power and available CSI (step 220). The process shown in FIG. 2may be performed at each transmission interval, which may correspond toa scheduling interval.

The transmission techniques described herein may also be used for othermulti-channel communication systems, such as OFDM systems, MIMO-OFDMsystems, and so on.

An OFDM system effectively partitions the system bandwidth into a numberof (N_(F)) frequency subchannels, which are also commonly referred to asfrequency bins or subbands. Each frequency subchannel is associated witha respective subcarrier (or frequency tone) upon which data may bemodulated. At each time slot, which is a particular time interval thatmay be dependent on the bandwidth of a frequency subchannel, amodulation symbol may be transmitted on each of the N_(F) frequencysubchannels. For the OFDM system, each frequency subchannel may bereferred to as a transmission channel, and there are N_(C)=N_(F)transmission channels for the OFDM system.

The frequency subchannels of the OFDM system may experience frequencyselective fading (i.e., different amounts of attenuation for differentfrequency subchannels). The specific response for the frequencysubchannels is dependent on the characteristics (e.g., the fading andmultipath effects) of the propagation path between the transmit andreceive antennas. Consequently, different effective SNRs may be achievedfor different frequency subchannels for a given amount of transmitpower. In this case, a particular transmission scheme may be selectedfor use for the N_(F) frequency subchannels in similar manner as thatdescribed above for the eigenmodes.

A MIMO-OFDM system includes N_(F) frequency subchannels for each of theN_(S) eigenmodes. Each frequency subchannel of each eigenmode may bereferred to as a transmission channel, and there are N_(C)=N_(F)·N_(S)transmission channels for the MIMO-OFDM system. The frequencysubchannels of each eigenmode in the MIMO-OFDM system may similarlyexperience different channel conditions and may achieve different SNRsfor a given amount of transmit power. In this case, a particulartransmission scheme may be selected for use for each of the N_(F)frequency subchannels in similar manner as that described above for theeigenmodes. However, since each transmit antenna is limited by a peaktransmit power of P_(max), the total transmit power used for allfrequency subchannels of a given transmit antenna is limited to P_(max).

For a MIMO-OFDM system, all transmission channels (i.e., for both thespatial and frequency dimensions) may be considered in determining thespecific transmission scheme to use for data transmission.Alternatively, the transmission scheme selection may be performed suchthat the transmission channels for only one dimension are considered atany given time.

The techniques described herein may also be used for groups oftransmission channels. Each group may include any number of transmissionchannels and may be associated with a respective operating point. Eachgroup may include, for example, the transmission channels to be used foran independent data stream, which may be associated with a particulardata rate and a particular coding and modulation scheme. For amultiple-access communication system, each group may be associated withthe transmission channels to be assigned to a different receiver.

For a wideband MIMO system that may experience frequency selectivefading, the operating bandwidth may be divided into a number of (equalor unequal) frequency bands such that each band may be considered as aflat-fading channel. In that case, each element of the channel responsematrix, H, behaves as a linear transfer function instead of a scalar,and the coupling, h_(i,j), between each transmit-receive antenna pairmay be represented by a vector of N_(F) scalar values, one scalar valuefor each frequency band. Various transmission schemes may be used for awideband MIMO system, some of which are described below.

In a first transmission scheme for the wideband MIMO system,beam-forming is used for each of the frequency bands. In oneimplementation of this first transmission scheme, the maximum eigenvalueis initially determined for each frequency band, and the eigenmodecorresponding to this eigenvalue is selected for use. The “optimal”power allocation for these eigenmodes may then be determined based onthe total transmit power, P_(tot), available at the transmitter for allN_(T) transmit antennas. This power allocation may be achieved usingwater-filling, as described in the aforementioned U.S. patentPublication No. US-2003-0072379-A1. N_(F) data streams may then beprocessed and transmitted on N_(F) selected eigenmodes of N_(F)frequency bands.

Depending on the channel response matrix, H, and the noise variance, σ²,each of the data streams transmitted on the N_(F) selected eigenmodesmay achieve different post-detection SNR. In one embodiment, each datastream is coded and modulated based on a respective coding andmodulation scheme selected for that data stream based on itspost-detection SNR. In another embodiment, the total transmit power,P_(tot), is allocated such that approximately similar post-detectionSNRs are achieved for all data streams. For this embodiment, one commoncoding and modulation scheme may be used for all data streams, which cansimplify the processing at both the transmitter and receiver.

Techniques for allocating transmit power to achieve similarpost-detection SNRs are described in the aforementioned U.S. patentPublication Nos. US-2003-0048856-A1 filed on May 17, 2001,US-2003-0112880-A1 filed Jun. 14, 2001, and U.S. Pat. No. 6,751,187filed Jun. 26, 2001 and issued Jun. 15, 2004. Techniques for processingdata at both the transmitter and receiver for a wideband MIMO system aredescribed in the aforementioned U.S. patent Publication No.US-2003-0125040-A1 and in U.S. Pat. No. 6,760,388, entitled “Time-DomainTransmit and Receive Processing with Channel Eigen-mode Decompositionwith MIMO Systems,” filed Dec. 7, 2001 and issued Jul. 6, 2004. Theseapplications are all assigned to the assignee of the present applicationand incorporated herein by reference.

In a second transmission scheme for the wideband MIMO system, thepartial-CSI transmission scheme is used for each of the frequency bands.In one implementation of this second transmission scheme, the peaktransmit power, P_(max), for each transmit antenna is divided equallyamong the N_(F) frequency bands. N_(S) data streams may then beprocessed and transmitted on each of the N_(F) frequency bands. In oneembodiment, each data stream may be coded and modulated based on itsachieved post-detection SNR. In another embodiment, to simplify thecoding and modulation at both the transmitter and receiver, the transmitpower may be allocated non-uniformly such that a common coding andmodulation scheme may be used for (1) all N_(S) data streams for eachfrequency band, (2) all N_(F) data streams for each spatial subchannel,(3) all data streams for all N_(S)·N_(F) transmission channels, or (4)all data streams for each group of transmission channels, which mayinclude any combination of frequency/spatial subchannels.

For each transmission scheme selected for use in the wideband MIMOsystem, the transmitter is provided with the information necessary toproperly process data prior to transmission. For example, for the firsttransmission scheme, which employs beam-forming for each of the N_(F)frequency bands, the transmitter may be provided with (1) N_(F) singularvectors for these N_(F) frequency bands and (2) information indicativeof the post-detection SNR for each of the N_(F) selected eigenmodes orthe coding and modulation scheme to be used for each data stream. Forthe second transmission scheme, which employs partial CSI transmissionfor each of the N_(F) frequency bands, the transmitter may be providedwith (1) the post-detection SNR for each data stream or each group ofdata streams.

Other transmission schemes that may be based on full-CSI or partial-CSIprocessing may also be used for the wideband MIMO system, and this iswithin the scope of the invention. These various transmission schemesallow the wideband MIMO system to consider the channel response of theindividual frequency bands for the data transmission.

In one embodiment, one full-CSI or partial-CSI based transmission schemeis selected for use for all frequency bands of the wideband MIMO system,as described above for the first and second transmission schemes. Thismay simplify the processing at both the transmitter and receiver. Inanother embodiment, each frequency band may be treated independently,and a different full-CSI or partial-CSI based transmission scheme may beselected for use for each frequency band, subject to certain constraintssuch as maintaining the total transmit power used for all frequencybands of a given transmit antenna to within the peak transmit power,P_(max).

For the wideband MIMO system, an equalizer may be employed at thereceiver and used to equalize the frequency selective fading in thechannel response. In this case, the transmission schemes available forthe narrowband MIMO system may be used for the wideband MIMO system.

In general, it is desirable to simplify the processing at both thetransmitter and receiver. This can be achieved by using as few codingand modulation schemes as possible (e.g., one coding and modulationscheme) for data transmission. One method for achieving this is toallocate the transmit power non-uniformly using selective channelinversion such that similar post-detection SNRs are achieved for thetransmission channels. An independent data stream may then be processed(e.g., based on a common coding and modulation scheme) and transmittedon each of these transmission channels. Alternatively, one data streammay be processed (e.g., based on the common coding and modulationscheme) and demultiplexed and transmitted over these multipletransmission channels. For example, selective channel inversion may beapplied in conjunction with the first transmission scheme describedabove, and one data stream may be processed and transmitted over theN_(F) selected eigenmodes of the N_(F) frequency bands.

Quantization of Channel State Information

The use of transmission schemes that rely on reduced amount of CSI(e.g., post-processed SNRs and singular vector) instead of full CSI(e.g., the channel response matrix) can greatly reduce the amount ofchannel information needed to be reported from the receiver to thetransmitter. A transmission scheme that relies on full CSI would requireN_(R)·N_(T) complex channel gains plus the noise variance, or theequivalent information, to be reported to the transmitter. In contrast,the partial-CSI transmission scheme only requires N_(S) SNR values,where N_(S)≦in {N_(T), N_(R)}. Each SNR value may be mapped to, andrepresented by, a particular “rate” that is supported by the MIMOsystem. The rate is indicative of the specific data rate and coding andmodulation scheme to be used for the data stream on the correspondingtransmit antenna.

The beam-forming transmission scheme only requires one SNR value for theselected eigenmode (or one rate value) and the singular vector, v, usedfor beam-forming the data transmission on the selected eigenmode. Thesingular vector is composed of N_(T) complex channel gains, one for eachtransmit antenna. If the number of bits used to quantize each real orimaginary dimension of each complex channel gain is denoted as Q, thenthe total number of bits needed to code the singular vector is 2N_(T)Q

Table 1 lists the number of bits needed to represent the different typesof CSI for the partial-CSI and beam-forming transmission schemes for aMIMO system with N_(T) transmit antennas and M possible rates. In Table1, the symbol “┌ ┐” denotes the next highest integer value of thequantity within the bracket. TABLE 1 Partial-CSI Beam-FormingCoding/Modulation N_(T)┌log₂(M)┐ ┌log₂(M)┐ Singular Vector 0 2N_(T)Q

In general, it is desirable to quantize the complex channel gains forthe singular vector to as few bits as possible to reduce the amount ofinformation to be reported back to the transmitter. However, thequantization should not noticeably degrade performance.

Table 2 shows the degradation in SNR (in dB) for different numbers ofquantization bits, Q, used to represent each dimension of a complexchannel gain of the singular vector. The degradation shown in Table 2 isobtained based on a 4×4 MIMO system operating in the beam-forming mode.The degradation in SNR due to the quantization is a function of thenumber of quantization bits only and is not a function of the operatingSNR. This degradation is shown versus for different values of Q rangingfrom two through five. TABLE 2 Number of Bits (Q) 2 3 4 5 Degradation inSNR (dB) 6.12 2.7 1.25 0.6As shown in Table 2, as low as four bits/dimension (i.e., Q=4), or eightbits/complex channel gain, may be used to encode the singular vector.Five bits/dimension (i.e., Q=5), or ten bits/complex channel gain value,provides even better performance. The number of bits required toadequately represent each complex channel gain may also be a function ofthe dimensionality of the MIMO system. For example, a 3×3 or 2×2 MIMOsystem may require even fewer than four or five bits per (real orimaginary) dimension of the complex channel gain.System

FIG. 3 is a block diagram of an embodiment of a transmitter system 310and a receiver system 350, which are capable of implementing variousaspects and embodiments described herein.

At transmitter system 310, traffic data is provided from a data source312 to a transmit (TX) data processor 314, which formats, codes, andinterleaves the traffic data based on one or more coding schemes toprovide coded data. The coded traffic data may then be multiplexed withpilot data using, e.g., time division multiplex (TDM) or code divisionmultiplex (CDM) in all or a subset of the transmission channels to beused for data transmission. The pilot data is typically a known datapattern processed in a known manner, if at all. The multiplexed pilotand coded traffic data is then modulated (i.e., symbol mapped) based onone or more modulation schemes (e.g., BPSK, QSPK, M-PSK, or M-QAM) toprovide modulation symbols. The data rate, coding, interleaving, andmodulation for each transmission channel or each group of transmissionchannels may be determined by various controls provided by a controller330.

A TX MIMO processor 320 may further process the modulation symbols inaccordance with a processing scheme corresponding to the currentoperating mode for transmitter system 310. Each transmission scheme maybe associated with a respective operating mode, and each operating modemay correspond to a specific processing scheme for the modulationsymbols. For the partial-CSI transmission scheme, TX MIMO processor 320simply passes the stream of modulation symbols for each data stream to arespective transmitter (TMTR) 322. For the beam-forming transmissionscheme, TX MIMO processor 320 pre-conditions the single stream ofmodulation symbols for the selected eigenmode based on the singularvector, v, associated with this eigenmode. The pre-conditioning may beperformed by multiplying each modulation symbol with each of the N_(T)entries for the singular vector to provide N_(T) scaled symbols. N_(T)streams of scaled symbols are thus provided for the N_(T) entries of thesingular vector. The pre-conditioning with the singular vectoreffectively performs beam-forming for the data stream. In either case,N_(T) scaled or unscaled modulation symbol streams are provided totransmitters 322 a through 322 t.

Each transmitter 322 receives and processes a respective symbol stream.For an OFDM system, each transmitter transforms the symbols (e.g., usinginverse fast Fourier transform (IFFT)) to form OFDM symbols, and mayfurther append a cyclic prefix to each OFDM symbol to form acorresponding transmission symbol. Each transmitter al converts thesymbol stream into one or more analog signals and further conditions(e.g., amplifies, filters, and quadrature modulates) the analog signalsto generate a modulated signal suitable for transmission over the MIMOchannel. N_(T) modulated signals from transmitters 322 a through 322 tare then transmitted from N_(T) antennas 324 a through 324 t,respectively.

At receiver system 350, the transmitted modulated signals are receivedby N_(R) antennas 352 a through 352 r, and the received signal from eachantenna 352 is provided to a respective receiver (RCVR) 354. Eachreceiver 354 conditions (e.g., filters, amplifies, and downconverts) thereceived signal and digitizes the conditioned signal to provide arespective stream of samples. Each sample stream may further beprocessed (e.g., demodulated with a recovered pilot) to obtain acorresponding stream of received symbols (denoted as y).

A RX MIMO processor 360 then receives and processes the N_(R) receivedsymbol streams based on one of a number of spatial receiver processingtechniques to provide N_(T) recovered symbol streams (denoted as r). Forexample, RX MIMO processor 360 may implement the CCMI, CCMI-SC, MMSE,MMSE-SC, or some other receiver processing technique. These variousreceiver processing techniques are described in detail in theaforementioned U.S. patent Publication No. US-2003-0125040-A1.

A receive (RX) data processor 362 then demodulates, deinterleaves, anddecodes the recovered symbols to provide the transmitted traffic data.The processing by RX MIMO processor 360 and RX data processor 362 iscomplementary to that performed by TX MIMO processor 320 and TX dataprocessor 314, respectively, at transmitter system 310.

RX MIMO processor 360 may further derive an estimate of the SNRs for thetransmission channels, the channel gains for the singular vectorcorresponding to the eigenmode with the highest eigenvalue (or bestSNR), and so on, and provide this information to a controller 370. RXdata processor 362 may also provide the status of each received frame orpacket, one or more other performance metrics indicative of the decodedresults, and possibly other information. Controller 370 collects thepertinent CSI, which may comprise all or some of the informationreceived from RX MIMO processor 360 and RX data processor 362. This CSIis then processed by a TX data processor 378, modulated by a modulator380, conditioned by transmitters 354 a through 354 r, and transmittedback to transmitter system 310.

At transmitter system 310, the modulated signals from receiver system350 are received by antennas 324, conditioned by receivers 322,demodulated by a demodulator 340, and processed by a RX data processor342 to recover the pertinent CSI reported by the receiver system. Thereported CSI is then provided to controller 330 and used to select thetransmission scheme and to generate various controls for TX dataprocessor 314 and TX MIMO processor 320.

Controllers 330 and 370 direct the operation at the transmitter andreceiver systems, respectively. Memories 332 and 372 provide storage forprogram codes and data used by controllers 330 and 370, respectively.

To implement the multi-mode transmission techniques described above,controller 330 receives the pertinent CSI from receiver system 350,which may include the post-processed SNRs, the singular vector, and/orsome other information descriptive of the characteristics or operatingcondition of the MIMO channel. Controller 330 then (1) selects aparticular transmission scheme to use for transmitting data, and (2)determines the rate to use for each selected transmission channel. Therate (i.e., the data rate and the coding and modulation scheme) to beused for each selected transmission channel is determined based in parton the amount of transmit power allocated to the data stream. The powerallocation and rate determination may also be performed by some networkentity rather than at the transmitter system.

The multi-mode transmission techniques described herein may beimplemented by various means. For example, these techniques may beimplemented in hardware, software, or a combination thereof. For ahardware implementation, the elements used to implement these techniquesmay be implemented within one or more application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), fieldprogrammable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described herein, or a combination thereof.

For a software implementation, certain aspects of the multi-modetransmission techniques may be implemented with modules (e.g.,procedures, functions, and so on) that perform the functions describedherein. The software codes may be stored in a memory unit (e.g., memory332 in FIG. 3) and executed by a processor (e.g., controller 330). Thememory unit may be implemented within the processor or external to theprocessor, in which case it can be communicatively coupled to theprocessor via various means as is known in the art.

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method for transmitting data over a plurality of transmissionchannels in a wireless communication system, comprising: determining anoperating condition of the communication system; identifying a specificpower allocation transmission scheme selected from among a plurality ofpossible power allocation transmission schemes based on the determinedoperating condition, wherein each of the possible power allocationtransmission schemes allocates a total transmit power available to thetransmitter among at least one of the plurality of transmissionchannels; determining one or more data streams to be transmitted basedon the selected transmission scheme; and processing the one or more datastreams based on the selected transmission scheme.
 2. The method ofclaim 1, wherein the specific transmission scheme is selected byevaluating performance of each of the plurality of possible transmissionscheme.
 3. The method of claim 1, wherein the specific transmissionscheme is selected based on an operatingsignal-to-noise-and-interference ratio (SNR).
 4. The method of claim 1,wherein the one or more data streams are further processed based onchannel-state information.
 5. The method of claim 4, wherein theplurality of transmission schemes includes a partial channel-stateinformation (CSI) transmission scheme.
 6. The method of claim 5, whereinthe channel-state information for the partial-CSI transmission schemecomprises signal-to-noise-and-interference ratios (SNRs).
 7. The methodof claim 4, wherein the plurality of transmission schemes include abeam-forming transmission scheme.
 8. The method of claim 7, wherein asingle transmission channel is selected for use for the beam-formingtransmission scheme, and wherein beam-forming is used for a datatransmission on the single selected transmission channel.
 9. The methodof claim 7, wherein the channel-state information for the beam-formingtransmission scheme comprises signal-to-noise-and-interference ratio(SNR) and a singular vector for a single selected transmission channel.10. The method of claim 9, wherein each element of the singular vectoris quantized to five bits or less per dimension.
 11. The method of claim4, wherein the plurality of transmission schemes include a beam-steeringtransmission scheme.
 12. The method of claim 11, wherein thechannel-state information for the beam-steering transmission schemecomprises signal-to-noise-and-interference ratio (SNR) and a vector ofphase values for a plurality of transmit antennas used for datatransmission.
 13. The method of claim 12, wherein the vector of phasevalues is based on a singular vector for a principal eigenmode.
 14. Themethod of claim 1, wherein the operating condition is quantified by asignal-to-noise-and-interference ratio (SNR), and wherein a partial-CSItransmission scheme is selected for use if an operating SNR of thecommunication system is above a threshold SNR, and a beam-formingtransmission scheme is selected for use if the operating SNR is belowthe threshold SNR.
 15. The method of claim 1, further comprising:selecting a rate for each data stream based on asignal-to-noise-and-interference ratio (SNR) achievable for the datastream.
 16. The method of claim 1, wherein the wireless communicationsystem is a multiple-input multiple-output (MIMO) communication systemand the plurality of transmission channels correspond to a plurality ofspatial subchannels of the MIMO communication system.
 17. The method ofclaim 1, wherein the wireless communication system is a widebandmultiple-input multiple-output (MIMO) communication system and theplurality of transmission channels correspond to a plurality of spatialsubchannels of a plurality of frequency bands.
 18. The method of claim17, wherein one transmission scheme is selected for use for allfrequency bands, and wherein one or more data streams are transmitted oneach frequency band and are processed based on the selected transmissionscheme.
 19. The method of claim 17, wherein one transmission scheme isselected for use for each frequency band, and wherein one or more datastreams are transmitted on each frequency band and are processed basedon the transmission scheme selected for that frequency band.
 20. Themethod of claim 17, wherein a beam-forming transmission scheme isselected for use for each of the plurality of frequency bands.
 21. Themethod of claim 20, wherein total transmit power is allocated such thata common coding and modulation scheme is used for all data streamstransmitted on the plurality of frequency bands.
 22. The method of claim17, wherein a partial-CSI transmission scheme is selected for use foreach of the plurality of frequency bands.
 23. The method of claim 22,wherein total transmit power is allocated such that a common coding andmodulation scheme is used for all data streams transmitted on eachfrequency band.
 24. The method of claim 22, wherein total transmit poweris allocated such that a common coding and modulation scheme is used forall data streams transmitted on each spatial subchannel.
 25. The methodof claim 1, wherein the wireless communication system is an orthogonalfrequency division multiplexing (OFDM) communication system and theplurality of transmission channels correspond to a plurality offrequency subchannels.
 26. A method for transmitting data on a pluralityof spatial subchannels in a multiple-input multiple-output (MIMO)communication system, comprising: determining an operatingsignal-to-noise-and-interference ratio (SNR) of the MIMO system;identifying a specific power allocation transmission scheme selectedfrom among a plurality of possible power allocation transmission schemesbased on the operating SNR, wherein each of the plurality oftransmission schemes is designated for use for a respective range ofoperating SNRs, and further wherein each of the plurality of possiblepower allocation transmission schemes allocates a total transmit poweravailable to the transmitter among at least one of the plurality ofspatial subchannels; determining one or more data streams to betransmitted based on the selected transmission scheme; and processingthe one or more data streams based on the selected transmission scheme.27. The method of claim 26, wherein the plurality of transmissionschemes include a partial-CSI transmission scheme and a beam-formingtransmission scheme, and wherein the partial-CSI transmission scheme isselected for use if the operating SNR is above a threshold SNR and thebeam-forming transmission scheme is selected for use if the operatingSNR is below the threshold SNR.
 28. A method for transmitting data in amultiple-input multiple-output (MIMO) communication system, comprising:identifying a specific one of a plurality of transmission channels basedon their achieved performance, wherein the specific channel is totransmit the total transmit power available to the transmitter;determining a vector of phase values corresponding to the selectedtransmission channel, one phase value for each of a plurality oftransmit antennas used for data transmission; processing data based on aparticular coding and modulation scheme; and transmitting the processeddata from each of the plurality of transmit antennas at a particulartransmit power and with a phase determined by the phase value associatedwith the transmit antenna.
 29. The method of claim 28, wherein fulltransmit power is used for each transmit antenna.
 30. A memorycommunicatively coupled to a digital signal processing device (DSPD)capable of interpreting digital information to: determine an operatingcondition of a communication system; select a specific power allocationtransmission scheme from among a plurality of possible power allocationtransmission schemes based on the determined operating condition,wherein a possible power allocation transmission scheme allocates atotal transmit power available to the transmitter among at least one ofa plurality of transmission channels; determine one or more data streamsto be transmitted based on the selected transmission scheme; and processthe one or more data streams based on the selected transmission scheme.31. A computer program product for facilitating data transmission over aplurality of transmission channels in a wireless communication system,comprising: code for determining an operating condition of thecommunication system; code for selecting a specific power allocationtransmission scheme from among a plurality of possible power allocationtransmission schemes based on the determined operating condition,wherein a possible power allocation transmission scheme allocates atotal transmit power available to the transmitter among at least one ofthe plurality of transmission channels; code for determining one or moredata streams to be transmitted based on the selected transmissionscheme; code for directing processing the one or more data streams basedon the selected transmission scheme; and a computer-usable medium forstoring the codes.
 32. An apparatus in a wireless communication system,comprising: means for determining an operating condition of thecommunication system; means for identifying a specific power allocationtransmission scheme selected from among a plurality of possible powerallocation transmission schemes based on the determined operatingcondition, wherein a possible power allocation transmission schemeallocates a total transmit power available to the transmitter among atleast one of a plurality of transmission channels; means for determiningone or more data streams to be transmitted based on the selectedtransmission scheme; and means for processing the one or more datastreams based on the selected transmission scheme.
 33. The apparatus ofclaim 32, wherein the wireless communication system is a multiple-inputmultiple-output (MIMO) communication system and the plurality oftransmission channels correspond to a plurality of spatial subchannelsof the MIMO communication system.
 34. The apparatus of claim 32, whereinthe specific transmission scheme is selected based on an operatingsignal-to-noise-and-interference ratio (SNR) of the communicationsystem, and wherein each of the plurality of transmission schemes isdesignated for use for a respective range of operating SNRs.
 35. Theapparatus of claim 34, wherein the plurality of transmission schemesinclude a partial-CSI transmission scheme and a beam-formingtransmission scheme, and wherein the partial-CSI transmission scheme isselected for use if the operating SNR is above a threshold SNR and thebeam-forming transmission scheme is selected for use if the operatingSNR is below the threshold SNR.
 36. The apparatus of claim 32, furthercomprising: means for selecting a rate for each data stream based on asignal-to-noise-and-interference ratio (SNR) achievable by the datastream.
 37. An apparatus in a multiple-input multiple-output (MIMO)communication system, comprising: means for determining an operatingsignal-to-noise-and-interference ratio (SNR) of the MIMO system; meansfor identifying a specific power allocation transmission scheme selectedfrom among a plurality of possible power allocation transmission schemesbased on the operating SNR, wherein each of the plurality oftransmission schemes is designated for use for a respective range ofoperating SNRs, wherein a possible power allocation transmission schemeallocates a total transmit power available to the transmitter among atleast one of a plurality of transmission channels; means for determiningone or more data streams to be transmitted based on the selectedtransmission scheme; and means for processing the one or more datastreams based on the selected transmission scheme.
 38. A controller in awireless communication system, comprising: means for receiving anoperating condition of the communication system; means for identifying aspecific power allocation transmission scheme selected from among aplurality of possible power allocation transmission schemes based on thedetermined operating condition, wherein a possible power allocationtransmission scheme allocates a total transmit power available to thetransmitter among at least one of a plurality of transmission channels;means for determining one or more data streams to be transmitted basedon the selected transmission scheme; and means for directing processingof the one or more data streams based on the selected transmissionscheme.
 39. The controller of claim 38, further comprising: means forselecting a rate for each data stream based on asignal-to-noise-and-interference ratio (SNR) achievable by the datastream.
 40. A base station comprising the controller of claim
 38. 41. Atransmitter unit in a wireless communication system, comprising: acontroller operative to direct data transmission over a plurality oftransmission channels by receiving an indication of an operatingcondition of the communication system, identifying a specific powerallocation transmission scheme selected from among a plurality ofpossible power allocation transmission schemes based on the operatingcondition, wherein a possible power allocation transmission schemeallocates a total transmit power available to a transmitter among atleast one of the plurality of transmission channels, determining one ormore data streams to be transmitted based on the selected transmissionscheme, selecting a rate for each data stream based in part on an amountof transmit power allocated to the data stream, and directing processingof the one or more data streams based on the selected transmissionscheme; a transmit (TX) data processor operative to process each datastream based on the selected rate to provide a respective stream ofsymbols; and one or more transmitters operative to process one or moresymbol streams to provide one or more modulated signals suitable fortransmission over a communication channel.
 42. The transmitter unit ofclaim 41, wherein the plurality of transmission schemes include apartial-CSI transmission scheme and a beam-forming transmission scheme.43. The transmitter unit of claim 42, wherein the controller isoperative to select the partial-CSI transmission scheme if an operatingsignal-to-noise-and-interference ratio (SNR) of the communication systemis above a threshold SNR and to select the beam-forming transmissionscheme if the operating SNR is below the threshold SNR.
 44. Thetransmitter unit of claim 42, wherein the controller is furtheroperative to utilize peak transmit power for each data stream for thepartial-CSI transmission scheme and to allocate all transmit power to asingle data stream for the beam-forming transmission scheme.
 45. Thetransmitter unit of claim 42, further comprising: a TX MIMO processoroperative to precondition the stream of symbols for a single data streambased on a singular vector for the beam-forming transmission scheme. 46.A base station comprising the transmitter unit of claim
 41. 47. Areceiver unit in a wireless communication system, comprising: a receive(RX) MIMO processor operative to receive and process a plurality ofstreams of received symbols in accordance with a particular receiverprocessing scheme to provide at least one stream of recovered symbols,and to derive channel-state information (CSI) for each recovered symbolstream; a RX data processor operative to process at least one recoveredsymbol stream in accordance with at least one demodulation and decodingscheme to provide decoded data; and a TX data processor operative toprocess the CSI for transmission back to a transmitter unit, and whereina specific power allocation transmission scheme is selected from among aplurality of possible power allocation transmission schemes based on theCSI, wherein one or more data streams are transmitted to the receiverunit based on the selected power allocation transmission scheme, andfurther wherein total available transmit power is allocated to the oneor more data streams based on the selected power allocation transmissionscheme.
 48. The receiver unit of claim 47, wherein the plurality oftransmission schemes include a partial-CSI transmission scheme and abeam-forming transmission scheme, and wherein the partial-CSItransmission scheme is selected if an operatingsignal-to-noise-and-interference ratio (SNR) is above a threshold SNRand the beam-forming transmission scheme is selected if the operatingSNR is below the threshold SNR.
 49. The receiver unit of claim 48,wherein for the beam-forming transmission scheme the RX MIMO processoris further operative to pre-condition the plurality of received symbolstreams with a singular vector to provide a single recovered symbolstream.
 50. The receiver unit of claim 48, wherein for the partial-CSItransmission scheme the RX MIMO processor is further operative toprocess the plurality of received symbol streams based on a minimum meansquare error with successive cancellation (MMSE-SC) receiver processingtechnique to provide a plurality of recovered symbol streams.
 51. Areceiver apparatus in a wireless communication system, comprising: meansfor processing a plurality of streams of received symbols in accordancewith a particular receiver processing scheme to provide at least onestream of recovered symbols, and to derive channel-state information(CSI) for each recovered symbol stream; means for processing the atleast one recovered symbol stream in accordance with at least onedemodulation and decoding scheme to provide decoded data; and means forprocessing the CSI for transmission back to a transmitter apparatus, andwherein a specific power allocation transmission scheme is selected fromamong a plurality of possible power allocation transmission schemesbased on the CSI, wherein one or more transmission data streams aretransmitted to the receiver apparatus based on the selected powerallocation transmission scheme, and wherein total available transmitpower is allocated to the one or more data streams based on the selectedpower allocation transmission scheme.